As is known, during the manufacture of laminar composite material structures, for example in carbon, a resin matrix is used to keep the fibres in place and give shape to the manufactured composite article. In this context, the formation of undesired pockets, or accumulations, of resin that would compromise the stress resistance of the article manufactured in this manner are sometimes observed. The formation process of resin pockets is the following (other types of pocket originate in a similar manner): when layers of composite material deform (orthogonally to the lamination plane) during manufacture, empty spaces or anomalous pressure conditions may generate. In particular, during the polymerization phase, resin in the liquid phase can fill these spaces, thus creating resin canals, or build-ups of resin, known as resin pockets, (parallel to the generatrix of the curved segment in the case of radial parts), which follow the profile of the first layer affected by the deformation. In some cases, the pockets can be filled by a material other than resin (adhesive, sealant, etc.).
The presence of the aforesaid resin pockets produces a wrinkling condition in which wrinkles can be formed both in the flat laminates and in curved zones (such as, for example, sections having a C, T, L, I or J shape). Non-destructive thermal, or thermographic, inspection methods for detecting these aforementioned defects, for example lock-in thermography, are known in the state of the art and used for detecting defects in a generic item or body under analysis. In fact, during heat stimulation, the defect in the body being tested reaches a different temperature from that reached by the surrounding material and/or the same temperature at different times (and which does not exhibit defects). This behaviour is due to the different thermo-physical properties involved in the heat-transmission phenomena, for example, the thermal conductivity of the material, the density of the material, etc.
In fact, if the item is opportunely stimulated, by means of suitable lamps, the presence of a defect creates a change in the normal heat diffusion, causing local lack of uniformity in surface temperature distribution on the item under investigation, easily identifiable through the use of a thermographic camera.
Thermographic technology is based on detecting the infrared radiation emitted from the body being tested, and exploits opportune sensors that work without having to be in contact with the body, determining the temperature and generating a thermal image.
The thermal image provided represents a map of the temperature distribution in the body being tested, from which it is possible to extract information regarding the internal structure of the body. The applications of thermographic technology cover the most varied sectors: from energy to infrastructure, from the transport and aeronautics industries to the conservation of cultural heritage and the medical industry. In the aeronautics field, thermography can be used as a non-destructive method for the periodic inspection of parts, to detect any defects that might arise during an aircraft's working life, or during the design and assembly stages.
The use of thermography in the non-destructive inspection sector is in rapid growth, linked to the considerable advantages this technology offers. Thermal maps can be obtained quickly and enable inspecting large surfaces without any contact with the structures being tested. This advantage results in a significant reduction in machine downtime, as no disassembly is necessary in order to perform the thermographic analysis. Lock-in thermography is applicable to the inspection of composite materials, for example carbon fibres, for which many classical technologies cannot be used.
On the other hand, traditional thermography is affected by some limits due to disturbances, such as non-uniform heating, reflections from heat sources in the working environment and non-homogeneous emissivity coefficients, which could be reproduced on the image of the item being tested and could therefore prejudice the result of the inspection.
For this reason, the processing of the acquired thermal signal currently represents an important field of investigation and study, for the purpose of identifying and separating the useful signal from the noise.
In lock-in thermography, temperature modulation induced by the heating system has a sinusoidal or square waveform, and propagates like a “thermal wave” inside the body being tested. This wave undergoes a series of reflections inside the body, such that the temperature modulation at the surface of the body is a function of the thermal wave that “returns” from inside the body.
The amplitude image of the wave reflected by the body is dependent on the surface's non-uniform absorption, infrared emission and the heating distribution, while the phase image is not particularly affected by these disturbances, generally resulting more reliable and sensitive. Furthermore, through phase analysis, the theoretical depth at which a defect (μ) can be detected is approximately twice that achievable with amplitude analysis and depends on the following formula:
  μ  =                              2          ⁢                                          ⁢          k                          ωρ          ⁢                                          ⁢                      C            p                                =                            2          ⁢          α                ω            
where: k is the heat conductivity of the material, ω is the angular velocity (equal to 2nf, where f is the frequency of the emitted sinusoidal wave), ρis the density, and Cp is the thermal capacity. The constant α represents the thermal diffusivity.
The emitted thermal wave can be obtained in various ways. For example, as shown in FIG. 1, one or more halogen lamps 3 connected to a signal generator 5 designed to generate a control signal (on/off) for the halogen lamps 3 variable in amplitude and frequency (for example, according to a sinusoidal or square waveform) could be used. In this way, one or more halogen lamps 3 generate, in use a modulated thermal wave, for example with square or sinusoidal wave modulation. Even more specifically, this modulated signal has a period of 25 seconds in this particular application.
A thermographic camera 8 acquires the thermal image of the item 4 being tested and sends the acquired data to a computer 7. The computer 7 acquires the signals coming from both the thermographic camera 8 and the trigger of the signal generator 5, which regulates the on/off phases of the halogen lamp 3. Specially provided software, stored in a memory of the computer 7, processes the data received by the computer 7 to generate phase and/or amplitude images of the thermal wave emitted from the body being tested.
Document WO 2014/044986 related to a method for assessing the depth of a crack in un metal material (a defect of the “vertical crack” type, also known in the state of the art as a “planar defect”). These defects are characterized by a vertical extension that is much larger than the extension in width, which is hence ignored (for this reason, the defect is considered two-dimensional). The method divulged in that document does not allow thermographic inspection for “volumetric” defects (i.e. defects where none of length, width and depth can be overlooked), as are resin pockets that occur in composite material structures, for example in glass fibre or carbon fibre.